1. Field of the Invention
The present invention is related to the field of vehicle suspensions and drivetrains. More specifically, the invention relates to semi-independent suspensions and drivetrains for vehicles.
2. Description of Related Art
Numerous designs for suspension and drivetrain systems are known and used in the manufacturing of various types of vehicles. It is known in vehicle engineering that particular designs provide specific advantages in particular applications. Most of the developments in the designing of suspension and drivetrain systems have been centered around automotive applications.
In recent times, smaller specialized all-terrain vehicles (a.k.a. ATVs) have gained in popularity as recreational and utility vehicles. As the popularity of ATVs has increases, so to have the performance demands placed upon them. Consequently, manufacturers of ATVs have responded with performance increases in certain areas, such as, increases in engine power and vehicle size. Such increases in engine output and vehicle size translate into increased inertial effects and extreme dynamic loading. These more powerful, massive ATVs usually require more skill and/or effort by the operator to maintain control during operation. However, ATV manufacturers have had very little success in modifying the previously mentioned automotive suspension and drivetrain designs to optimally adapt them for ATV use.
ATVs require the development of specialized suspension and drivetrain systems to improve operator controllability while continuing to withstand the rugged demands of their off-road application. Typically, ATVs have one or two front wheels and two rear wheels axially mounted on a solid axle in a dependent manner by a swing arm that pivots about a transverse axis of the ATV. Such a system is illustrated in U.S. Pat. No. 4,582,157 to Watanabe. The limitation and disadvantage of this suspension and drivetrain design is that the two rear wheels are mounted on a solid axle, which is axially coupled to a swing arm in such a way that it is only allowed to pivot about, and constrained to be parallel with, the transverse axis of the ATV.
Currently, the three and four wheeled ATVs using the '157's design, yield three undesirable characteristics that have negative effects on vehicle stability. The first two of these undesirable characteristics are in effect during both forward and turning or cornering operations of the ATV. These two characteristics are termed in vehicle engineering as suspensions having a roll center at ground level and possessing infinite roll resistance.
Having the roll center at ground level results in poor roll stability because the center of gravity (a.k.a. CG) of the vehicle can only be designed far above the vehicle's longitudinally oriented roll axis, potentially resulting in increased dynamic roll moment (a.k.a. torque). Infinite roll resistance implies that the suspension doesn't incorporate any roll motions to absorb roll energies. This means that all of the energy that is transferred via the unsprung mass (a.k.a. unsprung weight) from dynamic roll loading, is transferred directly into rolling the sprung mass (a.k.a. sprung weight) of the vehicle. Thus, infinite roll resistance translates into a harsh ATV dynamic roll response that is often difficult to predict and control by the operator. Even during simple forward motion, operating such an ATV can be like riding a twisting, bucking bronco when traversing uneven off-road terrain.
The third undesirable characteristic comes into play when the solid axle drivetrain of the '157 patent is used and the operator is attempting to negotiate the ATV to turn or comer. For the operator to negotiate the ATV around a turn, a sufficient turning moment must be generated by the operator to overcome all resistive turning moments. Usually, these resistive turning moments are primarily caused by inertial effects, which are overcome by the operator simply turning the front steering mechanism of the ATV. This steering action imparts the needed centripetal reaction from the front tires to overcome the inertial turning moments that work to maintain forward motion of the ATV.
This third undesirable characteristic, which is imparted due to the solid axle constraining the rear wheels to rotate at the same speed, is a mechanical counteracting turning moment, and it's contribution is only present while both rear tires are in sufficient traction with the terrain. This mechanical counteracting turning moment causes the ATV to experience a condition termed as understeer. For the operator to better negotiate this ATV to turn, one must overcome this understeer effect. This is typically accomplished by the operator leaning outward to shift the CG of the sprung mass such that a sufficient roll moment is imparted to cause the inside rear tire to lose traction with the terrain, thereby decoupling the mechanical counteracting turning moment caused by the solid axle. Thus, the operator must perilously put the ATV in an unsafe and unstable inertia induced roll condition in order to eliminate or reduce the mechanically induced understeer effect.
The sudden removal of this counteracting turning moment results in a nearly instantaneous transition from a quasi-static understeer condition to a sharp oversteer condition. This oversteer works to worsen the preexisting unstable inertia induced roll condition. Depending on the skill and strength of the operator, this situation can result in a rapid loss of operator roll control and vehicle rollover.
In light of the disadvantages inherent in the above suspension and drivetrain system, it has been recognized that significantly improved vehicle roll dynamics could be obtained if the rear suspension was designed such that the rear axle could also pivot about the vehicle's longitudinally oriented roll axis. These types of semi-independent suspensions offer variably finite roll resistance characteristics which are desirable for increased roll stability and traction.
The most important function of any suspension is to keep the tires in contact with the ground, while maximizing stability. Semi-independent rear suspension motion is all that is necessary for off-road ATV applications because the tires used are of low pressure, and they have rounded shoulders with radical tread patterns extending well into the sidewall region. These tire characteristics nullify the need of having a fully independent suspensions because the tires provide good compliance and traction with the terrain, even if the motion of one side of the suspension moderately affects the other.
In this regard, U.S. Pat. No. 5,845,918 to Grinde et al. discloses an ATV with a semi-independent rear suspension which allows the rear axle to pivot about the vehicle's longitudinal centerline as well as about a transverse axis. This suspension design has been found to substantially improve handling performance of the ATV by giving improved traction on uneven terrain and increased vehicle roll stability. In particular, during cornering, these semi-independent suspensions help roll stability because they postpone the initiation of the transition from understeer to oversteer. For the operator, a quasi-static understeer condition is easier to control than the rapid transition condition to a sharp oversteer.
The suspension system of the '918 patent however, does not totally resolve the third undesirable characteristic explained above, since it too uses a solid rear axle. In addition, the suspension design of the '918 patent severely limits the travel of the rear axle since the travel is limited by the travel of the coil-over shocks, which are displaced in near one to one ratio with the displacement of the rear axle. Thus, the suspension disclosed in the '918 patent is undesirable for ATV applications, especially for high performance applications, where amount of travel in a suspension is considered critical for optimal traction, energy absorption, and operator control.
Furthermore, the drivetrain of the '918 patent is like the other prior art suspension and drivetrain systems which typically utilize a drive shaft with a final drive bevel gear which are housed in a shaft housing and a final drive housing. As can be easily appreciated, these components are all made of metal and are quite massive thereby adding to the unsprung mass of the ATV.
Increased mass translates into power robbing inertial drivetrain losses, poor suspension response, and decreased overall power to mass (a.k.a, power to weight) ratio, which is very critical in high performance racing applications where maximum acceleration is imperative. More specifically, it is important to minimize the unsprung mass so that wheel hop frequencies are much higher than the sprung mass natural frequencies. This helps to ensure that the sprung mass remains relatively stable during wheel hop. Thus, a lesser unsprung mass provides superior suspension response and vehicle handling characteristics.
Lastly, because of the bulkiness of the suspension components and the presence of the drive shaft housing and the final drive housing in the prior art designs, there is no effective manner for providing a cost effective precision braking system for the rear wheels. In particular, it is well recognized that disk brake systems are especially desirable in high performance applications. Generally, disk brake systems provide more precise braking control than drum brake systems and are less massive, thus again, minimizing the unsprung mass and drivetrain inertial effects.
However, because the drive shaft housing and the final drive housing are generally positioned substantially center of the rear axle in a conventional ATV, they pose severe packaging constraints for a disk braking system. Thus, many ATVs incorporate the easier to package, yet less precise and more massive drum brakes at the outward ends of the axle housing, the only place possible for robust braking. Having these added braking masses outward from the central region of the axle further worsens the unsprung mass dynamic roll response by increasing the unsprung mass radius of gyration (a.k.a. polar moment of inertia).
One method of reducing the unsprung mass and reduce the bulkiness of the drivetrain is to utilize a chain and sprocket drive coupling such as those used in motorcycles, where they have proven to be superior to all other methods of drivetrain coupling for off-road applications. Chains and sprockets are less massive as compared to drive shafts and final drive bevel gears, and they provide a very responsive coupling of the drive wheels to the transmission. They also take up only minimal amount of space and impose only minimal packaging constraints for a disk braking system. Further, flexible couplings, including chains, absorb drivetrain shock, in the form of strain energy, providing a smoother coupling than that provided by shaft and gear drivetrain systems which often induce shock themselves because of gear lash issues.
However, the use of a conventional chain and sprocket drive system does not allow the rear axle to pivot about the vehicle's longitudinally oriented roll axis, for these chain couplings require that their elements remain planar. These conventional chain drive system typically incorporate a drive sprocket which is attached to the transmission and is in a fixed orientation, and a driven sprocket which spins about the drive axis and is constrained to pivot about, and remain parallel to, the rear transverse axis.
In other applications, special sprockets have been designed to allow the use of a chain and sprocket drivetrain while providing some amount of roll movement. Such drive sprockets are illustrated in U.S. Pat. No. 4,469,188 to Mita which is directed to an articulated tricycle including a drivetrain with a drive sprocket located about a shaft with a constant velocity universal joint employed to allow some flexibility between the sprocket and the shaft. The driven sprocket of the '188 patent is coupled to the solid rear axle by a chain for driving the rear wheels such that the front body of the tricycle may roll slightly relative to a rear body of the tricycle. However, application of the chain and sprocket drivetrain of the '188 patent has been found to be very difficult and inadequate in applications where large suspension travel and low unsprung mass are desired such as in an ATV application. The rear wheels of the '188 patent are supported primarily through the constant velocity universal joint housing and is inadequately supported for off road use. Furthermore, relative to the embodiment being discussed, the '188 patent, again, does not totally resolve the third undesirable characteristic explained above, since it too uses a solid rear axle, albeit because the two rear wheels are so close together and are so small, the resistance will be smaller than the other aforementioned prior art. Moreover, there are no easy ways to provide for the superior characteristics of a disk braking system.
U.S. Pat. No. 4,877,102 to Stewart discloses a multi-wheeled vehicle suspension and drive mechanism for ATVs including a rear axle assembly which allows the rear axle to roll. The '102 patent also discloses a sprocket and chain drive system including a driven sprocket with a universal joint which is mounted to the axle and aligned with the drive sprocket by a pivot arm which is mounted to the swing arm. Whereas the suspension and drive mechanism of the '102 patent allowed larger suspension travel and roll than the design of the '188 patent, the design disclosed in the '102 patent is complicated, requiring many numerous components. In particular, because the design disclosed in the '102 patent includes an axle housing and its associated components, which are all quite massive, and they counteract some of the benefits of using a chain drive in the first place since all of these additional components act to increase unsprung mass. In addition, because of the complexity, the design disclosed in the '102 patent is cost prohibitive to manufacture. Furthermore, because of the relative complexity of the system, it has been found to be unreliable, especially since dirt and debris tended to accumulate in the various components of the universal joint as well as the other exposed components. Lastly, it still fails to resolve the third undesirable characteristic explained above, since it too uses a solid rear axle. Consequently, this suspension and drive mechanism has not been readily accepted and is not commonly used.
Thus, despite the many disadvantages and limitations of commonly used ATV suspensions and drivetrains, they remain in use because there have yet to be any known practical alternatives which will practically avoid the aforementioned undesirable characteristics. Further, these commonly used rear suspension and drivetrain systems are accepted because some offer the required large range of suspension travel needed for added ground clearance and energy absorption. They are simple, tough, and packaged to minimize effects of collision with ground debris.
There are many other suspension and drive designs that could offer improved roll stability characteristics but at the expense of decreased suspension travel, reduced available ground clearance, and less energy absorption ability. These on-road, automotive type suspension systems are optimally suited for street applications where flat faced lower profile tires are used. Further, these designs are more complex, massive, and require packaging that is more vulnerable to collision with ground debris.
For the foregoing reasons, there exists an unfulfilled need for an improved semi-independent suspension and drivetrain system for vehicles which will enable improved roll and traction performance by allowing the axle to pivot about a vehicle's longitudinally oriented roll axis as well as a transverse axis. In addition, there exists an unfulfilled need for such a suspension and drivetrain system which will allow extensive range of suspension travel. Furthermore, there exists an unfulfilled need for such a suspension and drivetrain which will minimize the resistive turning moments associated with the usage of a solid rear axle. Still further, there exists an unfulfilled need for such a suspension and drivetrain which will enable the use of a, proven to be superior, flexible coupling drivetrain, such as a flexible chain coupling drivetrain, including a drive sprocket and a driven sprocket. Moreover, there exists an unfulfilled need for such a suspension and drivetrain which will attain the above objectives and include provisions for a disk brake system. Lastly, there exists an unfulfilled need for such a suspension and drivetrain which is simple, compact, robust, and cost effective.